A ball screw includes a screw shaft, a nut, and a plurality of balls interposed between the screw shaft and the nut (refer to Patent Literature 1, for example). A helical ball rolling groove where the balls roll is formed on an outer peripheral surface of the screw shaft. A helical loaded ball rolling groove facing the ball rolling groove of the screw shaft is formed on an inner peripheral surface of the nut. Since agile movement can be obtained by rolling motion of the balls, the ball screw is used as a machine element adapted to convert rotational movement to linear movement or convert linear movement to rotational movement.
The nut is provided with a return path connected between one end and the other end of the loaded ball rolling groove of the nut in order to circulate the balls. The return path constitutes part of a circulation path. When the screw shaft is rotated relative to the nut, the balls interposed between the screw shaft and the nut roll therebetween. A ball having rolled to the one end of the loaded ball rolling groove of the nut enters the return path, and returns to the other end of the loaded ball rolling groove of the nut after passing through the return path. Then, the ball rolls to the one end of the loaded ball rolling groove again.